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Geothermal energy is a concept that utilizes the difference between the surface of the earth and subsurface temperatures in order to generate (usually) steam generated power. In some cases water is pumped down into the earth where is it converted to steam and that steam is recovered and used to drive turbines to make electricity. Geothermal energy generation is a highly stable and low risk clean energy option. Some people believe that a geothermal grid is both the simplest and safest option for providing a clean global power solution as it is more reliable and efficient than a number of other clean energy technologies. It has been calculated that there is enough geothermal energy available that it could potentially replace all other options, potentially nuclear also, as both a viable and consistent source of power generation.

Using the earth as a heat source has advantages aside from being able to generate electricity:

Heating for homes and offices.

Heating water for pools, showers, etc.

Thermal control of soils used for crop production.

Drying materials such as timber and wool.

Thermal control of water used for fish-farming.

Normally the energy used for such thermal control is generated from less renewable resources including coal & gas power stations. Using geothermal energy simply for heating alone can save precious resources and allow for the freeing up some of the existing load on current energy grids.

The most commonly used technology, are used when the source hydrothermal water is not hot enough to drive a generator. Heat is transferred from the hydrothermal water to another liquid that has a lower boiling point than that of water, generating vapor that can drive a turbine.

Flash steam plants

These pull superheated water from deep below the surface into lower-pressure tanks to produce steam. They are most common in areas where high-temperature resources are available.

The water is pumped down through a injection well where it flows through fractures in the hot rocks.

It rises to the surface into a recovery well, from here it can be converted to steam.

A geothermal brine is combined with steam and transported from the recovery well(s) to a centralized separation station.

The steam / brine solution is separated and piped through these moisture separators to heat exchangers.(In the steam heat exchangers, the steam is cooled under pressure into condensate.)

At this stage the steam can be sent to steam turbines to make electricity. Waste steam is released through a steam exhaust.

The heat is then transferred to cold fresh water in condensate heat exchangers. The condensate cools down in the process to 20 degrees C.

Separated brine transfers heat to cold fresh water via specialized heat exchangers. (Geothermal brine is mineral-rich and causes scaling to coat the pipes in the heat exchanger. The solution to this is straightforward: steel particles are allowed to circulate in the stream, continually cleaning the pipes of any scaling buildup.)

Cold water is then pumped from wells to yet another storage tank.

From the storage tank, it is pumped to the heat exchangers where its temperature is increased.

It is then passed through deaerators where it is boiled at low vacuum pressure to remove dissolved oxygen and other gases that cause corrosion after being heated. This decrease in pressure cools the water in the process.

Hawaii There is one plant in Hawaii on Puna (the big island) and provides up to 30% if the island's power requirements.

Iceland Iceland is perhaps the premier user of and leader in geothermal energy worldwide with up to 89% of it's heating and energy requirements being sourced geothermally. Natural hot springs in Iceland have been popular for centuries so it would appear that the Icelandic people have in fact been taking advantage of this natural resource for centuries.